The present invention relates to a method of inspecting a spark plug and a method of manufacturing the spark plug.
A related art spark plug normally has an insulator having an axial hole that extends along an axis, a center electrode accommodated in the axial hole of the insulator with a part of the center electrode being exposed from a top end side of the insulator, a metal shell provided on an outer periphery of the insulator so as to cover a periphery of at least a part of the insulator, and a ground electrode connected to a top end side of the metal shell and bent so as to form a spark discharge gap between the ground electrode and the center electrode. By applying a high voltage to the center electrode of the spark plug, a spark discharge occurs between a tip of the center electrode and the ground electrode.
In a case of this kind of spark plug, however, if a crack appears or exists at the insulator, a discharge (called a “through-discharge”) occurs from the center electrode to the metal shell through its cracking portion of the insulator, and there is a risk that a normal discharge will not occur between the center electrode and the ground electrode. Therefore, in Japanese Unexamined Patent Application No. 2017-027726 (hereinafter is referred to as “JP2017-027726”), a method of inspecting such through-discharge is introduced as an inspection process into a manufacturing method of the spark plug.
As the inspection method disclosed in the JP2017-027726, an inspection high voltage is applied to the center electrode in a state in which the ground electrode is not bent yet, and a judgment is made as to whether or not there is a possibility that the through-discharge has occurred between the center electrode and the metal shell. This judgment is carried out by continuously detecting the voltage applied to the center electrode and comparing a differentiated value, which is obtained by differentiating this detected value of the applied voltage with respect to time, to a threshold value. Then, when an absolute value of the differentiated value is equal to or greater than the threshold value, it is judged that the through-discharge has occurred.